In a variety of marine environments, seismic surveys can be taken to gain a better understanding of geological formations beneath a body of water. Relatively large marine regions can be surveyed by a surface vessel or vessels towing seismic streamer cables through the water. Another vessel, or the same vessel, can be employed in providing a seismic source, such as a compressed air gun utilized to generate acoustic pulses in the water. The seismic source is used to generate energy that propagates down through the water and into the geological formation. Marine survey data on the geological formation can be obtained by detecting the energy reflected from interfaces between geological formations. Hydrophones are connected along the seismic streamer cables to detect the reflected energy.
Accurate collection of data by the hydrophones is affected by changes in characteristics of the water column, such as changes in sound velocity between regions of the water column in the survey area. The travel time of the reflected energy/signal through the water column is needed to accurately establish, for example, the depth of the target reflecting surface. Noting these changes are important in many situations such as, for example, when processing seismic data with three-dimensional (3D) and four-dimensional (4D) data processing, which includes 3D seismic data acquired at different times over the same area.
In some applications, sound velocity probes are dropped with varying frequency from a survey vessel during the seismic survey to collect data on sound velocity. The usefulness and frequency of the drops, however, can be limited by several factors, including operational safety considerations, risk of tangling the sound velocity probe line with the seismic spread equipment, requirements of the survey client, type of survey (e.g., 4D versus 3D), knowledge of the survey space, and time rate of sound velocity variation. Thus, sound velocity measurements are taken infrequently compared to the frequency of sound velocity change in time and space. However, in some situations probes may be used to measure sound velocity continuously from a moving vessel. Regardless, doing so still puts the probe, which is sometimes referred to as a “fish”, close to the towed seismic array as the fish approaches the water surface where the fish and/or its cable can tangle with the seismic spread equipment.